Wave filter



June 28, 1960 M. D. BRILL 2,943,280

WAVE FILTER Filed May 31, 1957 INVENTOR By M. 0. BRILL WfW ATTORNEYUnited States PatentO phone Laboratories, IncorporateQNew York, N.Y. a;

corporation ofiNew York y Filed May 31, 1957, SenNo. 662,623 s c1.3334-13 This invention relates to wave transmission networks and moreparticularly to a wave-guide filter of the varying-impedance type withimpedancecorrecting means.

The principal object of the invention-is to improve the impedance anddecrease the insertion loss in the transmission band of a waveguidefilter'of the varying-impedance type. A further objectis to maintainhigh attenuation in" the suppression-band of thefilter while reducingthe loss in the transmission band.

A waveguide has a natural high-pass cut-E frequency. If a low-passstructure with a higher cut-off frequency is associated with the waveguide, a band-pass filter is obtained. "The low-pass; structure may be afilter of-the varying-impedance type built into'the wave guide. Thefiltencomprises tandem-connected sections of guide having characteristicimpedances which are alternately lower and higher, or alternately higherand lower, than the terminal impedances between which the filter isdesigned to operate. If the wave guide is of the hollow-pipe type, witha rectangular cross section and a fixed width, the desired impedancesmay be obtained by making the height alternately less and greater thanthe height of the guide into which .the filter is inserted. Band-passwave-guide filters of this type are disclosed in Very High-FrequencyTechniques, compiled by the Stafl of the Radio Research Laboratory ofHarvard University and published by McGraw-Hill Book Company, Inc., NewYork and London, 1947, volume II, pages 731 to 736.

In such a filter, each change in height of the wave guide produces afield disturbance. transverse electric mode (TE which is ordinarilyused, each discontinuity is equivalent to the parasitic capacitive shuntsusceptance. Within the filter, the effects of these susceptances may beannulled by modifying the lengths of the low-impedance sections ofguide. The discontinuity susceptances at the ends of the filter,however, are not amenable to this treatment. Their presenceincreascs'the loss and degrades the impedance match in the transmissionband of the filter.

In accordance with the present invention, these end susceptances arecompensated for, and the impedance match in the band improved, by addingan annulling susceptance at one or both ends of the filter.- Thesepreferably take the form ofv capacitive screws or probes inserted intothe guide through a wider wall. For best performance, the probes aresymmetrically placed at points optimally spaced from each end of thefilter. In order to prevent the generation of unwanted highersordermodes, which would decrease the attenuation in the suppression region ofthe-filter, each susceptance comprises a pair of probes projectinginwardly from a wider wall of the wave guide in the same transverseplane and each spaced from a side wall approximately one-third of thewidth of the section.

The nature of the invention and its various objects,

For the dominant 1 ice pedance-correctingnaeans in accordance with thepresent invention.

h 1ow-pass fi e 1 s buil into awave g ded. ha ing a rectangular crosssection of width a and height I). The wider walls of the guide 2v areformed by an; nppenblock 3 and a lower block 4. The narrower side wallsare constituted by the plates 61 and 7 whichmay be secured to the blocks3 and 4 in any suitable manner. Electromagnetic waves are introducedinto the guide 2 at one :end, as indicated by the arrow 8, passthrough-the filter and arep pagatcdtn a suitable load, as indi a ed bythe arrow 9.

' The filter 1 comprises seven tandern connected sections.

of wave guide, numbered 11 through. 17, having char,-

a'cteristic irnpedances which are alternately lowerand' the dominantmode at some frequency f explained in t e abovcre e enc t fi t rl s desned as aicw: p ss st uc re. with a cut tr q en v f2 hi h r: han-f1- Theconrbination of wave guide and filter thus a band-pass characteristicwith the transmission band, eggtending' between and f 7 v 1 p A changein the heig t-of a r angul r wave g ide as from b to c or from): o d isqu a ntto a par 's shunt capacitive susceptance. {Each of the parasiticuss n ance app a n be we n tw jacent sta on of the fil er 'anfl c mpensed-by prop r y djust n th lengths of thev associated low-impedancesection, For example, the increase in height from c to d at the junctionof the sections 11 and 1.2 may be taken care of by modifying the lengthof section 1 1.- However, the-dis,- continuity between the guide '2and-the low-impedance sections 11 and '17 at, the ends of the filter 1can not be treated in this way.

In accordance with the present invention, impedance- -c.orrecting means.are added to take care of the end parasitic susceptances and also toimprove the impedance match between the filter 1 and the guide 2 in thetrans mission band. As shown, these means comprise a pair of probes orscrews 20, ZI-at the input end and a similar pair 22, 23 at the outputend. It will be understood that a single pair, at either end of thefilter l ,'may he sed, ut the tw pa provide symmet requ r compensationat each end, and give a better impedance match between the filter andthe guide. Each of the probes 20, 21, 22, and 23 projects inwardlythrough a tapped hole in ,a avider wall 3,015 the guide 2. The

I probes forming each pair are in the same transverse plane,

wave-glide filter of the varying-impedance type with imare spaced adistances from the end of the filter 1, and have approximately equalprojections.

There exist many combinations of probe penetration and distance s thatwill provide perfect ,intpeda-ncem tch .at a particular frequency. Ihecombination that nsua lly results in the best impedance match over aspecified re.- ai of t er p ba is tha co ina ion. o pen trati n anddistanc pro di g p rf ct m h at th c nter r qu ncy, is the r ion, w he atp be penetration; "The following relations apply .this'case. I

'Themagnitude, and the angle 6, of the complex reflection coefficient atthe frequency .f atone end of the uncorrected low-pass filter 1 whenterminated in the impedance of the guide Zjrnay be calculated ormeasured. For a perfect impedance gnatch at f,,, the minimum no ma zed.sys sptauce, h mlet a h pair. of

probes, corresponding to minimum penetration, is given by the expressionWhen b,,,,,,' is known, the required penetration may be determined.

The distance s is found from where A; is the wavelength in the guide 2at f and n is an integer. The value of n is chosen to make s equal Themaximum internal height, d, of the filter is usually madesufiicientlysmall so that over thefrequency range of interest as few as possible ofthe higher order modes excited by the discontinuities of the filter willpropagate. However, the mode cut-off frequencies for the higher order TEmodes, where m is greater than one, depend not on the height but only onthe width a of the filter 1 and the guide 2. The dimension a is alwayslarge enough to allow the propagation of the higher order TE modes, ifthese modes are excited by the incident energy. The transmissioncharacteristic of the filter forthese higher modes would be considerablydifierent than for the assumed dominant mode. In particular, theattenuation in 'thesuppression band may belowered considerably. Thegeometry of the filter 1 discourages the conversion of energy of thedominant mode into these higher order TE modes, but, unlessprecautionsare taken, the impedance-correeting means may perform such a conversion.Therefore, as a further important feature of the invention, the centerof each of the probes of a pair is spaced athird of the width a from anarrower side of the guide 2. Thus, the probe 20 is spaced a/3 from theside 6 and the probe 21 a/ 3 from the side 7. The space between theprobes is also a/3. The probes will react as shunt capacitance to thedominant mode but will not couple this mode to any TE modes where m iseven or any odd multiple of three. There will be no coupling to the evenmodes because of the symmetry of the probes with respect to the,transverse plane, and no coupling when m is an odd multiple of threebecause the probes 20 and 21 are located at points where the electricfields of these modes are zero. Therefore, the first higherorder TE modeto which energy of the incident dominant mode will couple is TE Theguide cut-off frequency for this mode is five times f and will almostalways be far out in the attenuating region of the filter 1 and in arelatively unimportant frequency range.

What is claimed is:

1. In combination, two end sections of rectangular wave guide havingunequal transverse dimensions, a low-pass wave filter insertedtherebetween, and impedance-correcting means associated with one of theend sections, the filter comprising a plurality of sections ofrectangular wave guide connected in tandem, the filter sections havingcharacteristic impedances which are alternately lower and higher thanthat of the end sections, said impedance-correcting means comprising apair of probes projecting inwardly from a wider Wall of the end sectionin the same transverse plane, and the centerof each of the probes beingspaced from a side of the end section a distance approximately equal toone-third of the width of the end section.

2. In combination, two endsections of rectangular Wave guide havingunequal transverse dimensions, a lowpass wave filter insertedtherebetween, and impedancecorrecting means associated with each of theend sections, the filter comprising a plurality of sections ofrectangular wave guide connected in tandem, the filter sections havingcharacteristic impedances which are alternately lower and higher thanthat of the end sections, each of the impedance-correcting meanscomprising a pair of probes projecting inwardly from a wider wall of theend section in the,same transverse plane, and the center of each of theprobes being spaced from a side of the end section a distanceapproximately equal to onethird of the width of the end section.

3. A band-pass wave filter comprising a plurality oftandem-connected'sections of rectangular wave guide having the samewidth and unequal transverse dimensions, the intermediatesections havingheights which are alternately less and greater than the height of theend sections, and impedance-correcting means comprising a pair of probesprojecting inwardly from a wider wall of one of the end sections in thesametransverse plane, the spacing between the probes and the distancebetween each probe and the nearer side of the'end section beingapproximately equal.

4. A band-pass wave filter comprising a plurality of tandem-connectedsections of rectangular wave guide having the same width and unequaltransverse dimensions, the intermediatesections having heights which arealternately less and greater than'the height of the end sections, andimpedance-correcting means associated with each of the end sections,each of the impedance-correcting means comprising a pair of probesprojecting inwardly from a wider wall of the end section in the sametransverse plane, and the spacing between the probes and the distancebetween each probe and the nearer side of the end section beingapproximately equal.

5. A band-pass wave filter comprising a plurality of tandem-connectedsections of rectangular wave guide having the same width and unequaltransverse dimensions, the intermediate sections having heights whichare alternately less and greater than the height of the end sec tions,and impedance-correcting means associated with each of the end sections,each of the impedance-correcting means comprising a pair of probesprojecting inwardly from a wider wall of the end section in the sametrans verse plane, and the plane being spaced from the adjoining sectiona distance s chosen to provide an impedance match between the endsection and the adjoining section at a selected frequency f in thetransmission band for a minimum penetration of the probes.

6. A filter in accordance with claim 5 in which s is equal at least toone-quarter but not more than threequarters of a wavelength at f, in theend section.

7. A filter in accordance with claim 5 in which the center of each ofthe probes is spaced from a side of the end section a distanceapproximately equal to onethird of the Width of the end section. v a

' 8. In combination, two end sections of rectangular wave guide havingunequal transverse dimensions, a wave transmission network insertedbetween the sections, the network comprising sections of rectangularwave guide and having a parasitic shunt susceptance at each end, andmeans associated with each of the end sections for compensating theparasitic shunt susceptance comprising a pair of probes projectinginwardly from a wider wall of the end section in the same transverseplane, the center of each of the probes being spaced from a side of theend section a distance approximately equal to one-third of the width ofthe end section.

References Cited inthe file of this patent UNITED STATES PATENTS OTHERREFERENCES Microwave Theory and Techniques, Reich et al. 1953 VanNostrand, pages 336-339 and 375-381.

